1. Field of the Invention
The present invention relates to a method of preparing a fuel cell including a proton conducting solid perovskite electrolyte membrane and a membrane electrode assembly of a fuel cell prepared by the method, and more particularly, to a method of preparing a fuel cell including a proton conducting solid perovskite electrolyte membrane with improved low temperature ion conductivity and a membrane electrode assembly of a fuel cell prepared by the method.
2. Description of the Related Art
Recently, as portable electronic devices and wireless communication devices become popularized, more research into fuel cells as a portable power supply device and a clean energy source is being conducted.
Fuel cells are power generation systems in which chemical energy is converted into electrical energy through electrochemical reaction of fuel gas such as hydrogen or methanol with an oxidant such as oxygen or air.
Fuel cells are classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells, alkali fuel cells, etc. according to the type of electrolyte used. Although theses fuel cells operate based on the same principle, they use different types of fuel, operational temperatures, catalysts, electrolytes, or the like.
Meanwhile, area specific resistance (ASR) of an electrolyte membrane follows the relation represented by Formula I with a thickness of the electrolyte membrane t and an absolute temperature T. Thus, in order to decrease resistance of the electrolyte membrane during the operation of the fuel cell, the thickness of the thin film t need be decreased or the operational temperature T need be increased to increase ion conductivity (σ).ASR[Ω*cm2]=t/σ(T)  Equation 1
where, temperature dependence of the ionic conductivity follows the Arrhenius relation represented by Equation 2 below.σ(T)*T=σ0*exp(−Ea/kT)  Equation 2
where, σ0 is a constant, k is Boltzmann constant, and Ea is activation energy.
Meanwhile, the solid oxide electrolyte has been used at a high temperature of 600° C. or higher since it has low ionic conductivity at a low temperature compared with other electrolytes such as polymer electrolyte. However, when the fuel cell is used at a high temperature, problems such as a long time to reach a high temperature, thermal expansion due to extreme temperature difference between operation and nonoperation and damage caused by the thermal expansion, reduction in durability of fuel cell material at a high temperature, and necessity of insulation due to difference between external and internal temperatures may occur.
ASR of commercialized fuel cells is about 0.1 Ω*cm2, and preferably not higher than 1.0 Ω*cm2. When a Nafion® polymer electrolyte is used, σ=0.1 S/cm at 80° C. Thus, when t=0.01 cm, ASR is 0.1 (0.01/0.1=0.1).
Meanwhile, according to FIG. 1 (K D Kreuer, Annu. Rev. Mater. Res., 33 (2003) 333), proton conductivity of BaY0.2Zr0.8O3-δ at 80° C. is 10−4.7=2.0*10−5 S/cm. In order to obtain a desired ASR (ASR=1.0 Ω*cm2), t needs to be 200 nm (t=ASR*σ=1.0×2.0×10−5=2.0*10−5 cm=200 nm). Thus, the electrolyte membrane needs to be formed as a thin film with a sub-micron thickness. Since the sub-micron thin film cannot be formed by a conventional ceramic process using powder, methods such as RF sputtering, magnetron sputtering, sol-gel spin coating, pulsed laser deposition and chemical vapor deposition are required to prepare a thin ceramic film.
In the preparation of a thin film, the solid oxide forms granular structure which has nano- to micro-meter grain size according to the processing conditions. Mostly, ion conductivity in grain boundaries is far less than that in bulk particles.
For example, FIG. 2 shows difference of proton conductivity of Y:BaZrO3 (BYZ in the following sentences) according to micro structures.
Referring to FIG. 2, the proton conductivity of an electrolyte membrane formed on a silicon substrate using pulsed laser deposition is about 500 times lower than that of a single crystalline bulk-structured electrolyte at 60° C. (2.38*10−8 S/cm vs. 1.34*10−5 S/cm).
The electrolyte membrane has such a low proton conductivity (σ) since the electrolyte membrane formed on a substrate using a solid oxide electrolyte material has a polycrystalline structure as shown in FIG. 3. That is, the proton conductivity (σ) is decreased since grain boundaries of the solid oxide electrolyte material act as barrier to the ion conduction in the electrolyte membrane having the polycrystalline structure. In FIG. 3, ions are conducted in the direction indicated by arrows.
ASR of 50 nm thick electrolyte membrane formed of solid oxide electrolyte with such a low ionic conductivity (σ) (i.e., 2.38*10−8 S/cm) will be 210 Ω*cm2 (50 nm/2.38*10−8 S/cm=50*10−7 cm/2.38*10−8 S/cm=210 Ω*cm2). Accordingly, the use of the electrolyte membrane in the fuel cell will result in excess resistance loss, which will prohibit any meaningful power generation. In order to decrease the ASR to a desired level, t needs to be 2.4 Å according to the equation ASR=t/σ(T) (t=1.0*2.38*10−8 cm=2.38*10−8 cm=2.38*10−1 nm=2.4 Å). However, since this value is less than a lattice constant of BYZ (4.19 Å), such a membrane cannot be formed. Accordingly, a fuel cell operating at a low temperature can be prepared only if ion conductivity of the electrolyte membrane formed by a thin film process can reach the same level of ion conductivity obtained in a bulk material.